Diesel engines may generally be classified as slow-speed, medium-speed or high-speed engines, with the slow-speed variety being used for the largest, deep draft vessels and in industrial applications. Slow-speed diesel engines are typically direct coupled, direct reversing, two-stroke cycle engines operating in the range of about 57 to 250 rpm and usually run on residual fuels. These engines are of crosshead construction with a diaphragm and stuffing boxes separating the power cylinders from the crankcase to prevent combustion products from entering the crankcase and mixing with the crankcase oil. Medium-speed engines typically operate in the range of 250 to about 1100 rpm and may operate on the four-stroke or two-stroke cycle. These engines are trunk piston design, and many operate on residual fuel as well. They may also operate on distillate fuel containing little or no residua. On deep-sea vessels these engines may be used for propulsion, ancillary applications or both. Slow speed and medium speed marine diesel engines are also extensively used in power plant operations. The present invention is applicable to them as well.
Each type of diesel engine employs lubricating oils to lubricate piston rings, cylinder liners, bearings for crank shafts and connecting rods, valve train mechanisms including cams and valve lifters, among other moving members. The lubricant prevents component wear, removes heat, neutralizes and disperses combustion products, prevents rust and corrosion, and prevents sludge formation or deposits.
In low-speed marine crosshead diesel engines, the cylinders and crankcase are lubricated separately, with cylinder lubrication being provided on a once-through basis by means of injection devices that apply cylinder oil to lubricators positioned around the cylinder liner. This is known as an “all-loss” lubrication system. The cylinder oil is typically formulated to provide for good oxidation and thermal stability, water demulsability, corrosion protection and good antifoam performance. Alkaline detergent additives are also present to neutralize acids formed during the combustion process. Dispersant, antioxidant, antifoam, antiwear and extreme pressure (EP) performance may also be provided by the use of suitable additives.
As engines produce higher power and are operated under more severe conditions, the lubricating oil's required functionality and performance have dramatically increased. These increased performance demands have resulted in a corresponding increase in the lubricant's expense. Lubricants are being made with increasingly sophisticated and expensive base stocks, including wholly synthetic base stocks. In addition, a wide variety of expensive additives, such as dispersants, detergents, antiwear agents, friction reducing agents, viscosity improvers, viscosity thickeners, metal passivators, acid sequestering agents and antioxidants are incorporated into the lubricants to meet functional demands.
Lubricants are designed to manage many engine condition parameters. One particularly important function of lubricating oils is to ensure the smooth operation of engines under every condition by limiting or preventing the wear and seizure of engine parts. Similarly, another engine condition parameter is the formation of carbonaceous type deposits, which is obviously undesirable and is managed by the lubricant's properties. The lubricant also manages other important engine condition parameters such as heat transfer, neutralization of combustion by-products, prevention of blowby, corrosion prevention, metal passivation and maintenance of lubricant film thickness. This list is not meant to be exhaustive and one of ordinary skill in the art recognizes many other important engine parameters managed by the lubricant.
No previous art has been identified concerning the modification of the properties of the incoming lubricant stream in response to engine conditions in an “all loss” system. However, various methods of replacing the lubricant or extending lubricant life in recirculating systems have been proposed. Those methods focused on maintaining a lubricant within known specification levels or replenishing additive concentration as opposed to actually responding to the real time or near real time needs of the operating engine.
Specifically, the previous art taught that when additive concentration levels in sump oil fell below a pre-set trigger, the engine was stopped and the entire lubricating oil was replaced. An improvement on this method allowed for large quantities of the sump oil to be removed and replaced with fresh lubricant during operation. Later practitioners improved upon this method to extend a recirculating lubricant's useful life by injecting additive into the sump when the additive concentrations had been depleted below a preset level.
The early methods of total or near total lubricant replacement were wasteful because they jettisoned many expensive components if only one additive concentration was lacking. These methods were further deficient in that the concentration of an additive did not necessarily correlate to the actual effectiveness of the lubricant inside the engine at any given point. Even if it did, substantial research has demonstrated that the concentration of the additive in the sump was not an accurate reflection of the additive concentration at the lubrication point of interest. See Malcolm Fox, et al., “Composition of Lubricating Oil in the Upper Ring Zone of an Internal Combustion Engine”, Tribology International, Vol. 24 No. 4, pp. 231-33 (August 1991). Therefore, these methods were not widely adopted as they did not ensure that the engine's actual lubrication needs would be fulfilled.